A spectrometry system in general includes an ionization apparatus (or ion source) for ionizing components of a sample of interest, an analyzer for separating the ions based on a discriminating attribute, an ion detector for counting the separated ions, and electronics for processing output signals from the ion detector as needed to produce user-interpretable spectral information. The spectral information may be utilized to determine the molecular structures of components of the sample, thereby enabling the sample to be qualitatively and quantitatively characterized. In a mass spectrometry (MS) system, the analyzer is a mass analyzer that separates the ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”). Depending on design, the mass analyzer may separate ions by utilizing electric and/or magnetic fields, or time-of-flight tubes. The mass analyzer is limited to operation at very low vacuum. In an ion mobility spectrometry (IMS) system, the analyzer is a drift cell that separates ions based on their different collision cross-sections. Ions are pulled through the drift cell by a DC voltage gradient in the presence of a drift gas. Ions of differing cross-sectional areas have differing mobilities through the gas environment. The drift cell may be configured for operation at vacuum or at atmospheric pressure. An IMS may be coupled with an MS to provide unique two-dimensional information about an analyte under investigation. Additionally, in certain “hyphenated” or “hybrid” systems the sample supplied to the ionization apparatus may first be subjected to a form of analytical separation. For example, in a liquid chromatography-mass spectrometry (LC-MS) system or a gas chromatography-mass spectrometry (GC-MS) system, the output of the LC or GC column may be transferred into the ionization apparatus through appropriate interface hardware.
Various types of ionization apparatuses may be utilized in conjunction with spectrometry. Some ionization apparatuses operate at vacuum levels while others operate at atmospheric pressure (AP). Each type of ionization technique has its advantages and disadvantages. For example, AP ionization is compatible with the high-throughput provided by LC instruments. Examples of AP ionization include, but are not limited to, electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) atmospheric pressure photo-ionization (APPI), or atmospheric-pressure laser ionization (APLI).
An AP interface is required to successfully couple an AP ionization apparatus to a spectrometer operating at vacuum such as an MS or a reduced-pressure IMS. One of the main areas of inefficiency of spectrometric analysis has to do with collecting ions from the AP ionization apparatus and transferring them into the lower pressure stages of the analyzer. The difficulty stems from the fact that ion motion in an AP interface is controlled by both electrostatic fields and, even more strongly, by the gas dynamics. Any attempts to focus ions into an ion transfer component such as an orifice or capillary through the use of electrostatic fields are hampered by the gas dynamics.
An existing approach to addressing this problem entails increasing the gas throughput of the AP interface so that more gas is admitted into the first differentially pumped stage of the spectrometer. This approach can increase the amount of ions captured as well. A disadvantage of this approach is that the separation of ions from the gas now must occur at a relative high pressure. Devices have been developed to provide such separation such as electrodynamic ion funnels. Such devices work reasonably well up to a certain pressure level but have their own disadvantages, including the addition of cost and complexity to the system. For example, additional pumps are usually needed to evacuate the gas. Moreover the gas dynamics at elevated pressures (such as 10 to 30 Torr) make focusing low mobility ions a challenge.
Therefore, there is an ongoing need for improvements in interfacing an atmospheric pressure ionization apparatus with a spectrometer.